To simplify hardware for processing image data by simultaneously reading both surfaces of a manuscript.

An image processing unit includes: readout means 11, 12 for simultaneously reading both surfaces of the manuscript; pixel rate conversion means 13, 14 for converting the pixel rates of the read front surface image data and the back surface image data into the same pixel rate; image shift means 16, 17 for adjusting deviation between a main scanning direction and a sub-scanning direction of the front surface image data and the back surface image data from the pixel rate conversion means 13, 14; an image composition means 18 for combining the front surface image data with the back surface image data from the image shift means 16, 17 on the same line; and an image processing means 19 for applying the same image processing to the front surface image data and the back surface image data combined in the image composition means 18.